Motors move toward more efficiency

Apr 1, 2008 9:32 AM, Leland Teschler Editor; Re-purposed with permission from Machine Design.

The next generation of motor-powered appliances and industrial machines will be energy misers thanks to new electronics and speed controls.

Cost comparison of energy efficient motors

It isn’t just speed control that is making appliances and other motion-based applications more efficient. Motors that excel at variable speeds just operate more efficiently than older models spinning at constant speeds. One indication of the difference comes from an analysis by International Rectifier Inc. which ranked losses of induction motors, synchronous reluctance motors, permanent-magnet synchronous motors, and internal permanent-magnet motors.

One problem with more efficient motors has been that the rare-earth magnets in their construction have been expensive. That situation started improving in the late 1990s and continues today. One analysis by IR compared the relative costs of components found in 750-W induction motors with those of an equivalent internal permanentmagnet motor. Using material costs as of 2006, the IPM came in cheaper by over $1/motor.

There are also subtleties to driving the most-efficient PM motors. Brushless-dc motors are happy with trapezoidal drive signals. On the other hand, PM synchronous motors and IPM motors have a sinusoidal back-EMF, and the drive signals must likewise be sinusoidal. It is only recently that motor-control platforms have had the necessary horse power to synthesize such signals.

The differences in drive signals stem largely from rotor construction. Ordinary PM motors have magnets on the surface of their rotor. In contrast, IPM motors put magnets inside slots on the rotor. This brings efficiencies in managing rotor flux at high speeds when field weakening comes into play.